Antenna System Planning and Practical Considerations 4-1

Chapter 4

Antenna System Planningand Practical Considerations

Selecting Your Antenn a System

Where should you start in putting together an antenna system? A newcomer to Amateur Radio,an amateur moving to a new location, or someone wanting to improve an existing “antennafarm” might ask this question. The answer: In a comfortable chair, with a pad and writing

instrument.The most important time spent in putting together an antenna system is that time spent in planning.

It can save a lot of time, money and frustration. While no one can tell you the exact steps you shouldtake in developing your master plan, this section, prepared by Chuck Hutchinson, K8CH, should helpyou with some ideas.

Begin planning by spelling out your communications desires. What bands are you interested in?Who (or where) do you want to talk to? When do you operate? How much time and money are youwilling to spend on an antenna system? What physical limitations affect your master plan?

From the answers to the above questions, begin to formulate goals—short, intermediate, and long range.Be realistic about those goals. Remember that there are three station effectiveness factors that are under yourcontrol. These are: operator skill, equipment in the shack, and the antenna system. There is no substitute fordeveloping operating skills. Some trade-offs are possible between shack equipment and antennas. For ex-ample, a high-power amplifier can compensate for a less than optimum antenna. By contrast, a better an-tenna has advantages for receiving as well as for transmitting.

Consider your limitations. Are there regulatory restrictions on antennas in your community? Arethere any deed restrictions or covenants that apply to your property? Do other factors (finances, familyconsiderations, other interests, and so forth) limit the type or height of antennas that you can erect? Allof these factors must be investigated because they play a major role determining the type of antennasyou erect.

Chances are that you won’t be able to immediately do all you desire. Think about how you canbudget your resources over a period of time. Your resources are your money, your time available towork, materials you may have on hand, friends that are willing to help, etc. One way to budget is toconcentrate your initial efforts on a given band or two. If your major interest is in chasing DX, youmight want to start with a very good antenna for the 14-MHz band. A simple multiband antenna couldinitially serve for other frequencies. Later you can add better antennas for those other bands.

SITE PLANNINGA map of your property or proposed antenna site can be of great help as you begin to consider

alternative antennas. You’ll need to know the size and location of buildings, trees and other majorobjects in the area. Be sure to note compass directions on your map. Graph paper or quadrille paper isvery useful for this purpose. See Fig 1 for an example. It’s a good idea to make a few photocopies ofyour site map so you can mark on the copies as you work on your plans.

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Use your map to plan antenna layouts andlocations of any supporting towers or masts. Ifyour plan calls for more than one tower or mast,think about using them as supports for wire an-tennas. As you work on a layout, be sure to thinkin three dimensions even though the map showsonly two.

Be sensitive to your neighbors. A 70-footguyed tower in the front yard of a house in a resi-dential neighborhood is not a good idea (and prob-ably won’t comply with local ordinances!).

ANALYSISUse the information in this book to analyze

antenna patterns in both horizontal and verticalplanes. If you want to work DX, you’ll want an-tennas that radiate energy at low angles. An an-tenna pattern is greatly affected by the presenceof ground. Therefore, be sure to consider whateffect ground will have on the antenna pattern atthe height you are considering. A 70-foot highantenna is approximately 1/2, 1, 11/2 and 2 wave-lengths (λ) high on 7, 14, 21 and 28 MHz respec-tively. Those heights are useful for long-distancecommunications. The same 70-foot height repre-sents only λ/4 at 3.5 MHz. Most of the radiated

energy from a dipole at that height would be concentrated straight up. This condition is not great forlong-distance communication, but can still be useful for DX work and excellent for short-range com-munication.

Lower heights can be useful for communication. However, it is generally true that “the higher, thebetter” as far as communications effectiveness is concerned.

There may be cases where it is not possible to install low-frequency dipoles at λ/4 or more abovethe ground. A vertical antenna with many radials is a good choice for long-distance communications.You may want to install both a dipole and a vertical for the 3.5 or 7-MHz bands. On the 1.8-MHz band,unless very tall supports are available, a vertical antenna is likely to be the most useful for DXing. Youcan then choose the antenna that performs best for a given set of conditions. A low dipole will gener-ally work better for shorter-range communications, while the vertical will generally be the better per-former over longer distances.

Consider the azimuthal pattern of fixed antennas. You’ll want to orient any fixed antennas to favorthe directions of greatest interest to you.

BUILDING THE SYSTEMWhen the planning is completed, it is time to begin construction of the antenna system. Chances

are that you can divide that construction into a series of phases or steps. Say, for example, that youhave lots of room and that your long-range plan calls for a pair of 100-foot towers to support monobandYagi antennas. The towers will also support a horizontal 3.5-MHz dipole at 100 feet, for DX work. Onyour map you’ve located them so the dipole will be broadside to Europe. Initially you decide to builda 60-foot tower with a triband beam and a 3.5-MHz inverted-V dipole to begin the project. In yourmaster plan, the 60-foot tower is really the bottom part of a 100-foot tower. The guys, anchors and allhardware are designed for use in the 100 footer.

Initially you buy a heavy-duty rotator and mast that will be needed for the monoband antennas

Fig 1—A site map such as this one is a usefultool for planning your antenna installation.

Antenna System Planning and Practical Considerations 4-3

Fig 2—Alternatives to a guyed tower are shown here. At A, the crank-up tower permits working onantennas at reduced height. It also allows antennas to be lowered during periods of no operation.Motor-driven versions are available. The fold-over tower at B and the combination at C permit workingon antennas at ground level.

later. Thus, you avoid having to buy, and then sell, a medium-duty rotator and lighter-weight towerequipment. You could have saved money in the long run by putting up a monoband beam for yourfavorite band, but you decided that for now it is more important to have a beam on 14, 21 and 28 MHz.The second step of your plan calls for installing the second tower. This time you’ve decided to waituntil you can install all 100 feet of that second tower, and put a 7-MHz Yagi on top of it. Later you willremove the top section of the first (60 foot) tower and insert the sections and add the guys to bring it upto 100 feet. You decide that at that time you’ll continue to use the tribander for a few months to seewhat difference the 60 foot to 100-foot height change makes.

COMPROMISESBecause of limitations, most amateurs are never able to build their “dream” antenna system. This

means that some compromises must be made. Do not, under any circumstances, compromise the safetyof an antenna installation. Follow the manufacturer’s recommendations for tower assembly, installa-tion and accessories. Make sure that all hardware is being used within its ratings.

Guyed towers are frequently used by radio amateurs because they cost less than more complicatedunguyed or freestanding towers with similar ratings. Guyed towers are fine for those who can climb, orthose with a friend who is willing to climb. But you may want to consider an antenna tower that foldsover, or one that cranks up (and down). Some towers crank up (and down) and fold over too. SeeFig 2. That makes for convenient access to antennas for adjustments and maintenance without climb-ing. Crank-up towers also offer another advantage. They allow antennas to be lowered during periodsof no operation, such as for aesthetic reasons or during periods of high winds.

A well-designed monoband Yagi should out-perform a multiband Yagi. In a monoband design the

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best adjustments can be made for gain, front-to-back ratio (F/B), and matching, but only for a singleband. In a multiband design, there are always trade-offs in these properties for the ability to operate onmore than one band. Nevertheless, a multiband antenna has many advantages over two or more singleband antennas. A multiband antenna requires less heavy duty hardware, requires only one feed line,takes up less space, and it costs less.

Apartment dwellers face much greater limitations in their choice of antennas. For most, the possi-bility of a tower is only a dream. (One enterprising ham made arrangements to purchase a top-floorcondominium from a developer. The arrangements were made before construction began, and the planswere altered to include a roof-top tower installation.) For apartment and condominium dwellers, thesituation is still far from hopeless. A later section presents ideas for consideration.

EXAMPLESYou can follow the procedure previously outlined to put together modest or very large an-

tenna systems. What might a ham put together for antennas when he or she wants to try a little ofeverything, and has a modest budget? Let’s suppose that the goals are (1) low cost, (2) no tower,(3) coverage of all HF bands and the repeater portion of one VHF band, and (4) the possibility ofworking some DX.

After studying the pages of this book, the station owner decides to first put up a 135 foot center-fedantenna. High trees in the backyard will serve as supports to about 50 feet. This antenna will cover allthe HF bands by using a balanced feeder and an antenna tuner. It should be good for DX contacts on10 MHz and above, and will probably work okay for DX contacts on the lower bands. However, herplan calls for a vertical for 3.5 and 7 MHz to enhance the DX possibilities on those bands. For VHF, achimney-mounted vertical is included.

ANOTHER EXAMPLEA licensed couple has bigger ambitions. Goals for their station are (1) a good setup for DX on 14,

21 and 28 MHz, (2) moderate cost, (3) one tower, (4) ability to work some DX on 1.8, 3.5 and 7 MHz,and (5) no need to cover the CW portion of the bands.

After considering the options, the couple decides to install a 65-foot guyed tower. A large commer-cial triband Yagi will be mounted on top of the tower. The center of a trap dipole tuned for the phoneportion of the 3.5 and 7-MHz bands will be supported by a wooden yard arm installed at the 60-footlevel of the tower, with ends drooping down to form an inverted V. An inverted L for 1.8 MHz startsnear ground level and goes up to a similar yard arm on the opposite side of the tower. The horizontalportion of the inverted L runs away from the tower at right angles to the trap dipole. Later, the husbandwill experiment with sloping antennas for 3.5 MHz. If those experiments are not successful, a λ/4vertical will be used on that band.

Apartment PossibilitiesA complete and accurate assessment of antenna types, antenna placement, and feed-line placement

is very important for the apartment dweller. Among the many possibilities for types are balcony anten-nas, “invisible“ ones (made of fine wire), vertical antennas disguised as flag poles or as masts with aTV antenna on top, and indoor antennas.

A number of amateurs have been successful in negotiating with the apartment owner or managerfor permission to install a short mast on the roof of the structure. Coaxial lines and rotator controlcables might be routed through conduit troughs or through duct work. If you live in one of the upperstories of the building, routing the cables over the edge of the roof and in through a window might bethe way to go. There is a story about one amateur who owns a triband beam mounted on a 10-foot mast.But even with such a short mast, he is the envy of all his amateur friends because of hissuperb antenna height. His mast stands on top of a 22-story apartment building.

Usually the challenge is to find ways to install antennas that are unobtrusive. That means searching

Antenna System Planning and Practical Considerations 4-5

out antenna locations such as balconies, eaves, nearby trees, etc. For example, a simple but effectivebalcony antenna is a dangling vertical. Attach an “invisible” wire to the tip of a mobile whip or a lengthof metal rod or tubing. Then mount the rigid part of the antenna horizontally on the balcony rail,dangling the wire over the edge. The antenna is operated against the balcony railing or other metallicframework. A matching network is usually required at the antenna feed point. Metal in the building willlikely give a directivity effect, but this may be of little consequence and perhaps even an advantage.The antenna may be removed and stored when not in use.

Frequently, the task of finding an inconspicuous route for a feed line is more difficult than theantenna installation itself. When Al Francisco, K7NHV, lived in an apartment, he used a tree-mountedvertical antenna. The coax feeder exited his apartment through a window and ran down the wall to theground. Al buried the section of line that went from under the window to a nearby tree. At the tree, asection of enameled wire was connected to the coax center conductor. He ran the wire up the side of thetree away from foot traffic. A few short radials completed the installation. The antenna worked fine,and was never noticed by the neighbors.

See Chapters 6 and 15 for ideas about low-frequency and portable antennas that might fit into youravailable space. Your options are limited as much by your imagination and ingenuity as by your pock-etbook. Another option for apartment dwellers is to operate away from home. Some hams concentrateon mobile operation as an alternative to a fixed station. It is possible to make a lot of contacts on HFmobile. Some have worked DXCC that way.

Suppose that you like VHF contests. Because of other activities, you are not particularly interestedin operating VHF outside the contests. Why not take your equipment and antennas to a hilltop for thecontests? Many hams combine a love for camping or hiking with their interest in radio.

Antennas for Limited SpaceIt is not always practical to erect full-size antennas for the HF bands. Those who live in apartment

buildings may be restricted to the use of minuscule radiators because of house rules, or simply becausethe required space for full-size antennas is unavailable. Other amateurs may desire small antennas foraesthetic reasons, perhaps to keep peace with neighbors who do not share their enthusiasm about hightowers and big antennas. There are many reasons why some amateurs prefer to use physically short-ened antennas; this chapter discusses proven designs and various ways of building and using themeffectively.

Few compromise antennas are capable of delivering the performance one can expect from the full-size variety. But the patient and skillful operator can often do as well as some who are equipped withhigh power and full-size antennas. Someone with a reduced-size antenna may not be able to “bore ahole” in the bands as often, and with the commanding dispatch enjoyed by those who are better equipped,but DX can be worked successfully when band conditions are suitable.

INVISIBLE ANTENNASWe amateurs don’t regard our antennas as eyesores; in fact, we almost always regard them as

works of art! But there are occasions when having an outdoor or visible antenna can present problems.When we are confronted with restrictions—self-imposed or otherwise—we can take advantage of

a number of options toward getting on the air and radiating at least a moderately effective signal. In thiscontext, a poor antenna is certainly better than no antenna at all! This section describes a number oftechniques that enable us to use indoor antennas or “invisible” antennas outdoors. Many of these sys-tems will yield good-to-excellent results for local and DX contacts, depending on band conditions atany given time. The most important consideration is that of not erecting any antenna that can presenta hazard (physical or electrical) to humans, animals and buildings. Safety first!

Clothesline AntennaClotheslines are sometimes attached to pulleys (Fig 3) so that the user can load the line and re-

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trieve the laundry from a back porch. Laundry lines of this variety are accepted parts of the neighbor-hood “scenery,” and can be used handily as amateur antennas by simply insulating the pulleys fromtheir support points. This calls for the use of a conducting type of clothesline, such as heavy gaugestranded electrical wire with Teflon or vinyl insulation. A high quality, flexible steel cable (stranded) issuitable as a substitute if one doesn’t mind cleaning it each time clothing is hung on it.

A jumper wire can be brought from one end of the line to the ham shack when the station is beingoperated. If a good electrical connection exists between the wire clothesline and the pulley, a perma-nent connection can be made by connecting the lead-in wire between the pulley and its insulator. Anantenna tuner can be used to match the “invisible” random-length wire to the transmitter and receiver.

Invisible Long WireA wire antenna is not actually a “long wire”

unless it is one wavelength or greater in length.Yet many amateurs refer to (relatively) long physi-cal spans of conductor as “long wires.” For thepurpose of this discussion we will assume we havea fairly long span of wire, and refer to it as an“end-fed” wire antenna.

If we use small-diameter enameled wire forour end-fed antenna, chances are that it will be verydifficult to see against the sky and neighborhoodscenery. The smaller the wire, the more “invisible”the antenna will be. The limiting factor with smallwire is fragility. A good compromise is #24 or #26magnet wire for spans up to 130 feet; lighter-gaugewire can be used for shorter spans, such as 30 or60 feet. The major threat to the longevity of finewire is icing. Also, birds may fly into the wire andbreak it. Therefore, this style of antenna may re-quire frequent service or replacement.

Fig 4 illustrates how we might install an in-visible end-fed wire. It is important that the insu-lators also be lacking in prominence. TinyPlexiglas blocks perform this function well. Small-diameter clear plastic medical vials are suitablealso. Some amateurs simply use rubber bands forend insulators, but they will deteriorate rapidlyfrom sun and air pollutants. They are entirely ad-equate for short-term operation with an invisibleantenna, however.

Rain Gutter and TV AntennasA great number of amateurs have taken ad-

vantage of standard house fixtures when contriv-ing inconspicuous antennas. A very old techniqueis the use of the gutter and down spout system onthe building. This is shown in Fig 5, where a leadwire is routed to the operating room from one endof the gutter trough. We must assume that the woodto which the gutter is affixed is dry and of goodquality to provide reasonable electrical insulation.

Fig 4—The “invisible” end-fed antenna.

Fig 3—The clothesline antenna is more than itappears to be.

Fig 5—Rain gutters and TV antenna installations canbe used as inconspicuous Amateur Radio antennas.

Antenna System Planning and Practical Considerations 4-7

Fig 6—A flagpole antenna.

The rain gutter antenna may perform quite poorly during wet weather or when there is ice and snow onit and the house roof.

All joints between gutter and down spout sections must be bonded electrically with straps of braidor flashing copper to provide good continuity in the system. Poor joints can permit rectification of RFand subsequently cause TVI and other harmonic interference. Also, it is prudent to insert a section ofplastic down spout about 8 feet above ground to prevent RF shocks or burns to passersby while theantenna is being used. Improved performance may result if the front and back gutters of the house arejoined by a jumper wire to increase the area of the antenna.

Fig 5 also shows a TV or FM antenna that can be employed as an invisible amateur antenna. Many ofthese antennas can be modified easily to accommodate the 144 or 222-MHz bands, thereby permittingthe use of the 300-Ω line as a feeder system. Some FM antennas can be used on 6 meters by adding #10bus wire extensions to the ends of the elements, and adjusting the match for an SWR of 1:1. If 300-Ω lineis used it will require a balun or antenna tuner to interface the line with the station equipment.

For operation in the HF bands, the TV or FM antenna feeders can be tied together at the transmitterend of the span and the system treated as a random length wire. If this is done, the 300-Ω line will haveto be on TV standoff insulators and spaced well away from phone and power company service entrancelines. Naturally, the TV or FM radio must be disconnected from the system when it is used for amateurwork! Similarly, masthead amplifiers and splitters must be removed from the line if the system is to beused for amateur operation. If the system is mostly vertical, a good RF ground system with manyradials around the base of the house should be used to improve performance.

A very nice top-loaded vertical can be made from a length of TV mast with a large TV antenna onthe top. Radials can be placed on the roof or at ground level with the TV “feed line” acting as part ofthe vertical. An extensive discussion of loaded verticals and radial systems is given in Chapter 6.

Flagpole AntennasWe can exhibit our patriotism and have an invisible amateur antenna at the same time by disguis-

ing our antenna as shown in Fig 6. The vertical antenna is a wire that has been placed inside a plasticor fiberglass pole.

The flagpole antenna shown is structured for a single amateur band, and it is assumed that the heightof the pole corresponds to a quarter wavelength for the chosen band. The radials and feed line can beburied in the ground as shown. In a practical installation, the sealed end of the coax cable would protrudeslightly into the lower end of the plastic pole.

If a large-diameter fiberglass pole were avail-able, a multiband trap vertical may be concealedinside it. Or we might use a metal pole and burya water-tight box at its base, containing fixed-tuned matching networks for the bands of inter-est. The networks could then be selected remotelyby means of relays inside the box. A 30-foot flag-pole would provide good results in this kind ofsystem, provided it was used in conjunction witha buried radial system.

Still another technique is one that employs awooden flagpole. A small diameter wire can bestapled to the pole and routed to the coax feederor matching network. The halyard could by it-self constitute the antenna wire if it were madefrom heavy duty insulated hookup wire. Thereare countless variations for this type of antenna,and they are limited only by the imagination ofthe amateur.

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Other Invisible AntennasSome amateurs have used the metal fence on apartment verandas as antennas, and have had good

results on the upper HF bands (14, 21 and 28 MHz). We must presume that the fences were not con-nected to the steel framework of the building, but rather were insulated by the concrete floor to whichthey were affixed. These veranda fences have also been used effectively as ground systems (counter-poises) for HF-band vertical antennas put in place temporarily after dark.

One New York City amateur uses the fire escape on his apartment building as a 7-MHz antenna,and reports good success working DX stations with it. Another apartment dweller makes use of thealuminum frame on his living room picture window as an antenna for 21 and 28 MHz. He works itagainst the metal conductors of the baseboard heater in the same room.

Many jokes have been told over the years about “bedspring antennas.” The idea is by no meansabsurd. Bedsprings and metal end boards have been used to advantage as antennas by many apartmentdwellers as 14, 21, and 28 MHz radiators. A counterpoise ground can be routed along the baseboard ofthe room and used in combination with the bedspring. It is important to remember that any independent(insulated) metal object of reasonable size can serve as an antenna if the transmitter can be matched toit. An amateur in Detroit once used his Shopsmith craft machine (about 5 feet tall) as a 28-MHz an-tenna. He worked a number of DX stations with it when band conditions were good.

A number of operators have used metal curtain rods and window screens for VHF work, and foundthem to be acceptable for local communication. Best results with any of these makeshift antennas willbe had when the “antennas” are kept well away from house wiring and other conductive objects.

INDOOR ANTENNASWithout question, the best place for your antenna is outdoors, and as high and in the clear as possible.

Some of us, however, for legal, social, neighborhood, family or landlord reasons, are restricted to indoorantennas. Having to settle for an indoor antenna is certainly a handicap for the amateur seeking effectiveradio communication, but that is not enough reason to abandon all operation in despair.

First, we should be aware of the reasons why indoor antennas do not work well. Principal faultsare: (1) low height above ground—the antenna cannot be placed higher than the highest peak of theroof, a point usually low in terms of wavelength at HF, (2) the antenna must function in a lossy RFenvironment involving close coupling to electrical wiring, guttering, plumbing and other parasitic con-ductors, besides dielectric losses in such nonconductors as wood, plaster and masonry, (3) sometimesthe antenna must be made small in terms of a wavelength and (4) usually it cannot be rotated. These areappreciable handicaps. Nevertheless, global communication with an indoor antenna is still possible,although you must be sure that you are not exposing anyone in your family or nearby neighbors toexcessive radiation. See Chapter 1 on Safety.

Some practical points in favor of the indoor antenna include: (1) freedom from weathering effectsand damage caused by wind, ice, rain and sunlight (the SWR of an attic antenna, however, can beaffected somewhat by a wet or snow-covered roof), (2) indoor antennas can be made from materialsthat would be altogether impractical outdoors, such as aluminum foil and thread (the antenna needsupport only its own weight), (3) the supporting structure is already in place, eliminating the need forantenna masts and (4) the antenna is readily accessible in all weather conditions, simplifying pruningor tuning, which can be accomplished without climbing or tilting over a tower.

EmpiricismA typical house or apartment presents such a complex electromagnetic environment that it is impossible

to predict theoretically which location or orientation of the indoor antenna will work best. This is wheregood old fashioned cut-and-try, use-what-works-best empiricism pays off. But to properly determine whatreally is most suitable requires an understanding of some antenna measuring fundamentals.

Unfortunately, many amateurs do not know how to evaluate performance scientifically or compare oneantenna with another. Typically, they will put up one antenna and try it out on the air to see how it “gets out”in comparison with a previous antenna. This is obviously a very poor evaluation method because there is no

Antenna System Planning and Practical Considerations 4-9

Fig 7—When antennas are compared on fadingsignals, the time delay involved in disconnectingand reconnecting coaxial cables is too long foraccurate measurements. A simple slide switchwill do well for switching coaxial lines at HF. Thefour components can be mounted in a tin can orany small metal box. Leads should be short anddirect. J1 through J3 are coaxial connectors.

way to know if the better or worse reports arecaused by changing band conditions, different S-meter characteristics, or any of several other fac-tors that could influence the reports received.

Many times the difference between two an-tennas or between two different locations foridentical antennas amounts to only a few deci-bels, a difference that is hard to discern unlessinstantaneous switching between the two is pos-sible. Those few decibels are not important un-der strong signal conditions, of course, but whenthe going gets rough, as is often the case withan indoor antenna, a few dB can make the dif-ference between solid copy and no possibilityof real communication.

Very little in the way of test equipment is needed for casual antenna evaluation, other than acommunications receiver. You can even do a qualitative comparison by ear, if you can switch antennasinstantaneously. Differences of less than 2 dB, however, are still hard to discern. The same is true of S-meters. Signal strength differences of less than a decibel are usually difficult to see. If you want thatlast fraction of a decibel, you should use a good ac voltmeter at the receiver audio output (with theAGC turned off).

In order to compare two antennas, switching the coaxial transmission line from one to the other isnecessary. No elaborate coaxial switch is needed; even a simple double throw toggle or slide switchwill provide more than 40 dB of isolation at HF. See Fig 7. Switching by means of manually connect-ing and disconnecting coaxial lines is not recommended because that takes too long. Fading can causesignal-strength changes during the changeover interval.

Whatever difference shows up in the strength of the received signal will be the difference inperformance between the two antennas in the direction of that signal. For this test to be valid, bothantennas must have nearly the same feed-point impedance, a condition that is reasonably well met ifthe SWR is below 2:1 on both antennas.

On ionospheric propagated signals (sky wave) there will be constant fading, and for a valid com-parison it will be necessary to take an average of the difference between the two antennas. Occasion-ally, the inferior antenna will deliver a stronger signal to the receiver, but in the long run the law ofaverages will put the better antenna ahead.

Of course with a ground-wave signal, such as that from a station across town, there will be nofading problems. A ground-wave signal will enable the operator to properly evaluate the antenna un-der test in the direction of the source. The results will be valid for ionospheric-propagated signals atlow elevation angles in that direction. On 28 MHz, all sky-wave signals arrive and leave at low angles.But on the lower bands, particularly 3.5 and 7 MHz, we often use signals propagated at high elevationangles, almost up to the zenith. For these angles a ground-wave test will not provide a proper evalua-tion of the antenna, and use of sky-wave signals becomes necessary.

DipolesAt HF the most practical indoor antenna is usually the dipole. Attempts to get more gain with parasitic

elements will usually fail because of close proximity of the ground or coupling to house wiring. Beamantenna dimensions determined outdoors will not usually be valid for an attic antenna because the roofstructure will cause dielectric loading of the parasitic elements. It is usually more worthwhile to spend timeoptimizing the location and performance of a dipole than to try to improve results with parasitic elements.

Most attics are not long enough to accommodate half-wave dipoles for 7 MHz and below. If this is thecase, some folding of the dipole will be necessary. The final shape of the antenna will depend on thedimensions and configuration of the attic. Remember that the center of the dipole carries the most current

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and therefore does most of the radiating. This partshould be as high and unfolded as possible. Becausethe dipole ends radiate less energy than the center,their orientation is not as important. They do carrythe maximum voltage, nevertheless, so care shouldbe taken to position the ends far enough from otherconductors to avoid arcing.

The dipole may end up being L-shaped, Z-shaped, U-shaped or some indescribable cork-screw shape, depending on what space is avail-able, but reasonable performance can often be hadeven with such a nonlinear arrangement. Fig 8shows some possible configurations. Multibandoperation is possible with the use of open-wirefeeders and an antenna tuner.

One alternative not shown here is the alumi-num-foil dipole, which was conceived by RudyStork, KA5FSB. He suggests mounting the dipolebehind wallpaper or in the attic, with portability,ease of construction and adjustment, and economyin design among its desirable features. This antennashould also display reasonably good bandwidth re-sulting from the large area of its conductor mate-rial. If coaxial feed is used, some pruning of an at-tic antenna to establish minimum SWR at the bandcenter will be required. Tuning the antenna outdoorsand then installing it inside is usually not feasiblesince the behavior of the antenna will not be thesame when placed in the attic. Resonance will beaffected somewhat if the antenna is bent.

Even if the antenna is placed in a straight line,parasitic conductors and dielectric loading by nearbywood structures can affect the impedance. Trap and

Fig 9—Ways to orient a pair of perpendiculardipoles. The orientation at A and B will result in nomutual coupling between the two dipoles, but therewill be some coupling in the configuration shownat C. End (EI) and center (CI) insulators are shown.

Fig 8—Various configurations for small indoor antennas. See text for discussion.

Antenna System Planning and Practical Considerations 4-11

loaded dipoles are shorter than the full-sized versions, but are comparable performers. Trap dipoles arediscussed in Chapter 7; loaded dipoles in Chapter 6.

Dipole OrientationTheoretically a vertical dipole is most effective at low radiation angles, but practical experience

shows that the horizontal dipole is usually a better indoor antenna. A high horizontal dipole does ex-hibit directional effects at low radiation angles, but you will not be likely to see much, if any, directiv-ity with an attic-mounted dipole. Some operators place two dipoles at right angles to each other withprovisions at the operating position for switching between the two. Their reasoning is the radiationpatterns will inevitably be distorted in an unpredictable manner by nearby parasitic conductors. Therewill be little coupling between the dipoles if they are oriented at right angles to each other as shown inFigs 9A and 9B. There will be some coupling with the arrangement shown in Fig 9C, but even thisorientation is preferable to a single dipole.

With two antennas mounted 90° apart, you may find that one dipole is consistently better in nearlyall directions, in which case you will want to remove the inferior dipole, perhaps placing it someplaceelse. In this manner the best spots in the house or attic can be determined experimentally.

Parasitic ConductorsInevitably, any conductor in your house near a quarter wave in length or longer at the operating

frequency will be parasitically coupled to your antenna. The word parasitic is particularly appropriate inthis case because these conductors usually introduce losses and leave less energy for radiation intospace. Unlike the parasitic elements in a beam antenna conductors such as house wiring and plumbingare usually connected to lossy objects such as earth, electrical appliances, masonry or other objects thatdissipate energy. Even where this energy is reradiated, it is not likely to be in the right phase in thedesired direction; it is, in fact, likely to be a source of RFI.

There are, however, some things that can be done about parasitic conductors. The most obvious is toreroute them at right angles to the antenna or close to the ground, or even underground—procedures that areusually not feasible in a finished home. Where these conductors cannot be rerouted, other measures can betaken. Electrical wiring can be broken up with RF chokes to prevent the flow of radio-frequency currentswhile permitting 60-Hz current (or audio, in the case of telephone wires) to flow unimpeded. A typical RFchoke for a power line can be 100 turns of #10 insulated wire close wound on a length of 1-inch diameterplastic pipe. Of course one choke will be needed for each conductor. A three-wire line calls for three chokes.The chokes can be simplified by winding them bifilar or trifilar on a single coil form.

THE RESONANT BREAKERObviously, RF chokes cannot be used on conductors such as metal conduit or water pipes. But it is

still possible, surprising as it may seem, to obstruct RF currents on such conductors without breakingthe metal. The resonant breaker was first described by Fred Brown, W6HPH, in Oct 1979 QST.

Fig 10 shows a method of accomplishing this. A figure-eight loop is inductively coupled to the

Fig 10—A “resonant breaker” such as shown herecan be used to obstruct radio-frequency currentsin a conductor without the need to break the con-ductor physicall y. A vernier dial is recommendedfor use with the variable capacitor because tuningis quite sharp. The 100-pF capacitor is in serieswith the loop. This resonant breaker tunes from 14through 29.7 MHz. Larger models may beconstructed for the lower frequency bands.

4-12 Chapter 4

parasitic conductor and is resonated to the desired frequency with a variable capacitor. The result is avery high impedance induced in series with the pipe, conduit or wire. This impedance will block theflow of radio-frequency currents. The figure-eight coil can be thought of as two turns of an air-coretoroid and since the parasitic conductor threads through the hole of this core, there will be tight cou-pling between the two. Inasmuch as the figure-eight coil is parallel resonated, transformer action willreflect a high impedance in series with the linear conductor.

Before you bother with a “resonant breaker” of this type, be sure that there is a significant amountof RF current flowing in the parasitic conductor, and that you will therefore benefit from installingone. The relative magnitude of this current can be determined with an RF current probe of the typedescribed in Chapter 27. According to the rule of thumb regarding parasitic conductor current, if itmeasures less than 1/10 of that measured near the center of the dipole, the parasitic current is generallynot large enough to be of concern.

The current probe is also needed for resonating the breaker after it is installed. Normally, theresonant breaker will be placed on the parasitic conductor near the point of maximum current. When itis tuned through resonance, there will be a sharp dip in RF current, as indicated by the current probe.Of course, the resonant breaker will be effective only on one band. You will need one for each bandwhere there is significant current indicated by the probe.

Power-Handling CapabilitySo far, our discussion has been limited to the indoor antenna as a receiving antenna, except for the

current measurements, where it is necessary to supply a small amount of power to the antenna. Thesemeasurements will not indicate the full power-handling capability of the antenna. Any tendency toflash over must be determined by running full power or, preferably, somewhat more than the peakpower you intend to use in regular operation. The antenna should be carefully checked for arcing or RFheating before you do any operating. Bear in mind that attics are indeed vulnerable to fire hazards. Apotential of several hundred volts exists at the ends of a dipole fed by the typical Amateur Radiotransmitter. If a power amplifier is used, there could be a few thousand volts at the ends of the dipole.Keep your antenna elements well away from other objects. Safety first!

Construction Details and Practical ConsiderationsUltimately the success of an antenna project depends on the details of how the antenna is fabri-

cated. A great deal of construction information is given in other chapters of this book. For example theconstruction of HF Yagis is discussed in Chapter 11, Quad arrays in Chapter 12, VHF antennas inChapter 18 and in Chapter 20 there is an excellent discussion of antenna materials, particularly wireand tubing for elements. Here is still more helpful antenna construction information.

END EFFECTIf the standard expression λ/2 ≈ 491.8/f(MHz) is used for the length of a λ/2 wire antenna, the

antenna will resonate at a somewhat lower frequency than is desired. The reason is that in addition tothe effect of the conductor diameter and ground effects (Chapter 3) an additional “loading” effect iscaused by the insulators used at the ends of the wires to support the antenna. The insulators and thewire loops that tie the insulators to the antenna add a small amount of capacitance to the system. Thiscapacitance helps to tune the antenna to a slightly lower frequency, in much the same way that addi-tional capacitance in any tuned circuit lowers the resonant frequency. In an antenna this is called endeffect. The current at the ends of the antenna does not quite reach zero because of the end effect, asthere is some current flowing into the end capacitance. Note that the computations used to create Figs2 through 7 in Chapter 2 did not take into account any end effect.

End effect increases with frequency and varies slightly with different installations. However, atfrequencies up to 30 MHz (the frequency range over which wire antennas are most commonly used),experience shows that the length of a practical λ/2 antenna, including the effect of diameter and end

Antenna System Planning and Practical Considerations 4-13

Fig 11—Some ideas for homemadeantenna insulators.

effect, is on the order of 5% less than the length of a half wave in space. As an average, then, thephysical length of a resonant λ/2 wire antenna can be found from:

λ = × ≈491.8 0.95f(MHz)

468f(MHz) (Eq 1)

Eq 1 is reasonably accurate for finding the physical length of a λ/2 antenna for a given frequency,but does not apply to antennas longer than a half wave in length. In the practical case, if the antennalength must be adjusted to exact frequency (not all antenna systems require it) the length should be“pruned” to resonance.

INSULATORSWire antennas must be insulated at the ends. Commercially available insulators are made from

ceramic, glass or plastic. Insulators are available from many Amateur Radio dealers. Radio Shack andlocal hardware stores are other possible sources. Acceptable homemade insulators may be fashionedfrom a variety of material including (but not limited to) acrylic sheet or rod, PVC tubing, wood, fiber-glass rod or even stiff plastic from a discarded container. Fig 11 shows some homemade insulators.Ceramic or glass insulators will usually outlast the wire, so they are highly recommended for a safe,reliable, permanent installation. Other materials may tear under stress or break down in the presence ofsunlight. Many types of plastic do not weather well.

InstallingTransmission Lines

Many wire antennas require an insulator at the feedpoint. Although there are many ways to connect the feedline, there are a few things to keep in mind. If you feedyour antenna with coaxial cable, you have two choices.You can install an SO-239 connector on the center insu-lator, as shown by the center example in Fig 12, and usea PL-259 on the end of your coax, or you can separatethe center conductor from the braid and connect the feedline directly to the antenna wire as shown in the othertwo examples in Fig 12 and the example in Fig 13. Al-though it costs less to connect direct, the use of connec-tors offers several advantages. Coaxial cable braid soaks

Fig 12—Some homemadedipole center insulators. Theone in the center includes abuilt-in SO-239 connecto r.Others are designed for directconnection to the feed line.

4-14 Chapter 4

Fig 13—Details of dipole antenna construction. At A, the end insulator connection is shown. At B, thecompleted antenna is shown. A balun (not shown) is often used at the feed point, since this is abalanced antenna.

Fig 14—A piece of cut Plexiglas can be used as acenter insulator and to support a ladder-linefeeder. The Plexiglas acts to reduce the flexing ofthe wires where they connect to the antenna. Usethick Plexiglas in areas subject to high winds.

up water like a sponge. If you do not adequatelyseal the antenna end of the feed line, water willfind its way into the braid. Water in the feed linewill lead to contamination, rendering the coax use-less long before its normal lifetime is up.

It is not uncommon for water to drip from theend of the coax inside the shack after a year or so ofservice if the antenna connection is not properlywaterproofed. Use of a PL-259/SO-239 combination(or connector of your choice) makes the task of wa-terproofing connections much easier. Another advan-tage to using the PL-259/SO-239 combination is thatfeed-line replacement is much easier, should that be-come necessary.

Whether you use coaxial cable, ladder line, ortwin lead to feed your antenna, an often overlookedconsideration is the mechanical strength of the con-nection. Wire antennas and feed lines tend to movea lot in the breeze, and unless the feed line is at-tached securely, the connection will weaken withtime. The resulting failure can range from a frus-trating intermittent electrical connection to a com-plete separation of feed line and antenna. Fig 13and Fig 14 illustrate different ways of attachingeither coax or ladder line to the antenna securely.

Antenna System Planning and Practical Considerations 4-15

When open-wire feed line is used, the conductors of the line should be anchored to the insulator bythreading them through the eyes of the insulator two or three times, and twisting the wire back on itselfbefore soldering. A slack tie wire should then be used between the feeder conductor and the antenna, asshown in Fig 14. (The tie wires may be extensions of the line conductors themselves.) When window-typeline is suspended from an antenna in a manner such as that shown in Fig 14, the line should be twisted—atseveral twists per foot—to prevent stress hardening of the wire because of constant flexing in the wind.

When using plastic-insulated open-wire line, the tendency of the line to twist and short out close tothe antenna can be counteracted by making the center insulator of the antenna longer than the spacingof the line, as shown in Fig 14. In severe wind areas, it may be necessary to use 1/4-inch thick Plexiglasfor the center insulator rather than thinner material.

RUNNING THE FEED LINE FROM THE ANTENNA TO THE STATIONChapter 24 contains some general guidelines for installing feed lines. More detailed information is

contained in this section. Whenever possible, the transmission line should be lead away from the an-tenna at a 90° angle to minimize coupling from the antenna to the transmission line. This coupling cancause unequal currents on the transmission line, which will then radiate and it can detune the antenna.

Except for the portion of the line in close proximity to the antenna, coaxial cable requires noparticular care in running from the antenna to the station entrance, other than protection from mechani-cal damage. If the antenna is not supported at the center, the line should be fastened to a post more thanhead high located under the center of the antenna, allowing enough slack between the post and theantenna to take care of any movement of the antenna in the wind. If the antenna feed point is supportedby a tower or mast, the cable can be taped to the mast at intervals or to one leg of the tower.

Coaxial cable rated for direct burial can be buried a few inches in the ground to make the run fromthe antenna to the station. A deep slit can be cut by pushing a square-end spade full depth into theground and moving the handle back and forth to widen the slit before removing the spade. After thecable has been pushed into the slit with a piece of 1-inch board 3 or 4 inches wide, the slit can betamped closed.

Solid ribbon or the newer “window” types of line should be kept reasonably well spaced from otherconductors running parallel to it for more than a few feet. The “rule of thumb” is to space open-wire lineaway from other conductors by at least twice the spacing between the wires in the line. TV-type standoffinsulators with strap clamp mountings can be used for running this type of line down a mast or tower leg.Similar insulators of the screw-in type can be used in supporting the line on wooden poles for a long run.

Open-wire lines with bare conductors require frequent supports to keep the lines from twisting andshorting out, as well as to relieve the strain. One method of supporting a long horizontal run of heavyopen-wire line is shown in Fig 15. The line must be anchored securely at a point under the feed point ofthe antenna. Window-type line can be supported similarly with wire links fastened to the insulators.

To keep the line clear of pedestrians and vehicles, it is usually desirable to anchor the feed line atthe eaves or rafter line of the station building (see Fig 16), and then drop it vertically to the point ofentrance. The points of anchorage and entrance should be chosen to permit the vertical drop withoutcrossing windows.

If the station is located in a room on the groundfloor, one way of bringing coax transmission lineinto the house is to go through the outside wallbelow floor level, feed it through the basement orcrawl space, and then up to the station through ahole in the floor. When making the entrance holein the side of the building, suitable measurementsshould be made in advance to be sure the hole willgo through the sill 2 or 3 inches above the founda-tion line (and between joists if the bore is parallelto the joists). The line should be allowed to sag

Fig 15—A support for open-wire line. Thesupport at the antenna end of the line must besufficiently rigid to stand the tension of the line.

4-16 Chapter 4

Fig 17—An adjustable window lead-in panel madeup of two sheets of Lucite or Plexiglas. A feed-through connector for coax line can be made asshown in Fig 18. Ceramic feedthrough insulatorsare suitable for open-wire line. (W1RVE)

Fig 18—Feedthrough connector for coax line. AnAmphenol 83-1J (PL-258) connector, the type usedto splice sections of coax line together, is solderedinto a hole cut in a brass mounting flange. AnAmphenol bulkhead adapter 83-1F may be usedinstead.

Fig 19—A simple lightning arrester for open-wireline made from three standoff or feedthroughinsulators and sections of 1/8×1/2-inch brass orcopper strap. It should be installed in the line atthe point where the line enters the station. Theheavy ground lead should be as short and asdirect as possible. The gap setting should beadjusted to the minimum width that will prohibitarcing when the transmitter is operated.

Fig 16—Anchoring open-wire line at the stationend. The springs are especially desirable if theline is not supported between the antenna andthe anchoring point.

Antenna System Planning and Practical Considerations 4-17

Fig 20—The lightning arrester of Fig 19 may beused with 300 -Ω ribbon line in the manner shownhere. The TV standoffs support the line an inch orso away from the grounded center member of thearreste r.

below the entrance hole level outside the building to allow rain water to drip off.Open-wire line can be fed in a similar manner, although it will require a separate hole for each

conductor. Each hole should be insulated with a length of polystyrene or Lucite tubing. If available,ceramic tubes salvaged from old-fashioned “knob and tube” electrical installations, work very well forthis purpose. Drill the holes with a slight downward slant toward the outside of the building to preventrain seepage. With window ladder line, it will be necessary to remove a few of the spreader insulators,cut the line before passing through the holes (allowing enough length to reach the inside), and splicethe remainder on the inside.

If the station is located above ground level, or if there is other objection to the procedure describedabove, entrance can be made at a window, using the arrangement shown in Fig 17. An Amphenol type 83-1F(UG-363) connector can be used as shown in Fig 18; ceramic feedthrough insulators can be used for open-wire line. Ribbon line can be run through clearance holes in the panel, and secured by a winding of tape oneither side of the panel, or by cutting the retaining rings and insulators from a pair of TV standoff insulators,and clamping one on each side of the panel.

LIGHTNING PROTECTIONTwo or three types of lightning arresters for coaxial cable are available on the market. If the an-

tenna feed point is at the top of a well-grounded tower, the arrester can be fastened securely to the topof the tower for grounding purposes. A short length of cable, terminated in a coaxial plug, is then runfrom the antenna feed point to one receptacle of the arrester, while the transmission line is run from theother arrester receptacle to the station. Such arresters may also be placed at the entrance point to thestation, if a suitable ground connection is available at that point (or arresters may be placed at bothpoints for added insurance).

The construction of a homemade arrester foropen-wire line is shown in Fig 19. This type ofarrester can be adapted to ribbon line an inch or soaway from the center member of the arrester, asshown in Fig 20. Sufficient insulation should beremoved from the line where it crosses the arresterto permit soldering the arrester connecting leads.

Lightning GroundsLightning-ground connecting leads should be

of conductor size equivalent to at least #10 wire.The #8 aluminum wire used for TV-antennagrounds is satisfactory. Copper braid 3/4 inch wide(Belden 8662-10) is also suitable. The conductorshould run in a straight line to the grounding point.The ground connection may be made to a waterpipe system (if the pipe is not plastic), the groundedmetal frame of a building, or to one or more 5/8-inch ground rods driven to a depth of at least 8feet. More detailed information on lightning pro-tection is contained in Chapter 1.